I don't know about thinning it out -- were you thinking of using another spec of tran oil?
If it were mine, I would do it and the ATF filter -- to "zero" the age of all the fluids you can - oil, coolant, ATF. Start a new maintenance log from that and track your changes.
When the fluid is drained from the transmission - you might as well inspect the gasket/ring seal on the multi-function/range switch connector -- mine suffered a slow leak from this.